Explosive Detection Systems (EDS) are used for detecting explosives and other contraband. They are used commonly in the airline industry and their prevalence and importance has increased after 9/11.
It is critically important that the technology used in EDS be sufficiently advanced so as not to miss the detection of explosives. Balanced with that, the technology should be sufficiently advanced so as to minimize false alarms and maximize throughput.
EDSs commonly use X-rays to penetrate an object of interest, such as a bag or container, which is placed on a conveyer belt and moved through the system. X-rays are emitted from an X-ray source and are directed at the object. Transmitted and/or reflected or refracted X-rays are detected by detectors. An image of the object is reconstructed from the detected X-rays and a threat detection is made, either manually by an operator who views the image, or automatically by a threat detection algorithm implemented in software.
The use of computed tomography (CT) scanners are known in the industry as a sensitive and accurate EDS, but typically have a lesser throughput. Advancements in CT EDS technology have improved throughput. A CT scanner is helpful in that it can determine the density of an object being observed. Determining the density can enable the system to decipher most explosives. There are, however, innocuous materials that are close in density to explosives, causing a high false alarm rate when basing the determination solely on density. Similarly, density alone is not sufficient information to decipher all explosives.
Dual energy CT scanners are known in the industry and enable the determination of Zeffective of an object of interest, which enables the determination of the material from which the object is made, in order to decipher explosives. In other words, determining the Zeffective of an object will enable one to discriminate it from objects of similar density, when density alone would not enable such discrimination.
Several approaches exist for the use of dual energy CT scanning. One such approach is employed in the L-3 Communications Examiner® EDS. The Examiner employs a dual energy X-ray source. A high-voltage power supply switches between a higher voltage (e.g., 160 Kv) and a lower voltage (e.g., 80 Kv). The power supply switches from the high voltage to the low voltage at a certain frequency which in turn causes the X-ray source to emit high energy X-rays and low energy X-rays at this frequency.
One drawback associated with this approach is the significant limitation on the frequency with which the power supply can switch from high to low and low to high. When switching from high to low, a sufficient amount of time must pass in order to enable the dissipation of the energy built up during the high-energy phase. Similarly, when switching from low to high, a sufficient amount of time must pass in order to build up the energy needed to obtain the high voltage required. Thus, present systems employing this approach have frequency limitations. One such system, the Multiview Tomography (MTV) system of L-3 Communications, can switch up to 240 times per second, well below the desired frequency of a few kHz for next generation CT scanners.
Another approach at dual energy CT scanning employs the use of two sets of detectors, each detector set sensitive to a different energy level. This approach uses one single energy X-ray source. As it is, CT scanners use multiple detectors. This approach would double the number of detectors, which results in several drawbacks: size, manufacturability, and cost, among them.